oracle学习笔记（一）

count（*）和Count（列）

drop table t purge;
create table t as select * from dba_objects;
--alter table T modify object_id  null;
update t set object_id =rownum ;
set timing on 
set linesize 1000
set autotrace on 

　　select count(*) from t;

 select count(object_id) from t;

 逻辑读和Cost是一样的。、

建索引：

create index idx_object_id on t(object_id);

select count(*) from t;  和以前一样。

select count(object_id) from t;

 把列指定为非空。

alter table T modify object_id  not  null;

select count(*) from t;

select count(object_id) from t;

 看来count(列)和count(*)其实一样快，如果索引列是非空的，count(*)可用到索引，此时一样快，有索引，列不允许为空，性能一样快。

验证脚本1 （先构造出表和数据）

SET SERVEROUTPUT ON
SET ECHO ON
---构造出有25个字段的表T
DROP TABLE t;
DECLARE
  l_sql VARCHAR2(32767);
BEGIN
  l_sql := 'CREATE TABLE t (';
  FOR i IN 1..25 
  LOOP
    l_sql := l_sql || 'n' || i || ' NUMBER,';
  END LOOP;
  l_sql := l_sql || 'pad VARCHAR2(1000)) PCTFREE 10';
  EXECUTE IMMEDIATE l_sql;
END;
/
----将记录还有这个表T中填充
DECLARE
  l_sql VARCHAR2(32767);
BEGIN
  l_sql := 'INSERT INTO t SELECT ';
  FOR i IN 1..25
  LOOP
    l_sql := l_sql || '0,';
  END LOOP;
  l_sql := l_sql || 'NULL FROM dual CONNECT BY level <= 10000';
  EXECUTE IMMEDIATE l_sql;
  COMMIT;
END;
/

--以下动作观察执行速度，比较发现COUNT(*)最快，COUNT(最大列）最慢

DECLARE
  l_dummy PLS_INTEGER;
  l_start PLS_INTEGER;
  l_stop PLS_INTEGER;
  l_sql VARCHAR2(100);
BEGIN
  l_start := dbms_utility.get_time;
  FOR j IN 1..1000
  LOOP
    EXECUTE IMMEDIATE 'SELECT count(*) FROM t' INTO l_dummy;
  END LOOP;
  l_stop := dbms_utility.get_time;
  dbms_output.put_line((l_stop-l_start)/100);

  FOR i IN 1..25
  LOOP
    l_sql := 'SELECT count(n' || i || ') FROM t';
    l_start := dbms_utility.get_time;
    FOR j IN 1..1000
    LOOP
      EXECUTE IMMEDIATE l_sql INTO l_dummy;
    END LOOP;
    l_stop := dbms_utility.get_time;
    dbms_output.put_line((l_stop-l_start)/100);
  END LOOP;
END;
/

　-结论：

--原来优化器是这么搞的：列的偏移量决定性能，列越靠后，访问的开销越大。由于count(*)的算法与列偏移量无关，所以count(*)最快。

 表的连接顺序：

drop table tab_big;
drop table tab_small;
create table tab_big  as select * from dba_objects where rownum<=30000;
create table tab_small  as select * from dba_objects where rownum<=10;
set autotrace traceonly
set linesize 1000
set timing on 
select count(*) from tab_big,tab_small   ;  
select count(*) from tab_small,tab_big   ;

　　以上实验发现性能是一样的。

*+rule*回到规则的时代

select /*+rule*/ count(*) from tab_big,tab_small ;  
select /*+rule*/ count(*) from tab_small,tab_big ;

　　上一条性能好于下一条

结论：原来表连接顺序的说法早就过时了，那是基于规则的时代，现在我们是基于代价的。

与表顺序条件有关：

drop table t1 purge;
drop table t2 purge;
create table t1 as select * from dba_objects;
create table t2 as select rownum id ,dbms_random.string('b', 50) n ,data_object_id data_id from dba_objects where rownum<=10000;
set autotrace traceonly
set linesize 1000
set timing on
select /*+rule*/ * from t1,t2 where t1.object_id=29 and t2.data_id>8;
select /*+rule*/ * from t1,t2 where t2.data_id>8 and t1.object_id=29 ;


加个关联条件看看，看看
select /*+rule*/ * from t1,t2 where t1.object_id=t2.id and t1.object_id=29 and t2.data_id>8;
select /*+rule*/ * from t1,t2 where t1.object_id=t2.id and t2.data_id>8 and t1.object_id=29 ;

　　in与exists(10g)

select * from v$version;

drop table emp purge;
drop table dept purge;
create table emp as select * from scott.emp;
create table dept as select * from scott.dept;
set timing on 
set linesize 1000
set autotrace traceonly 
select * from dept where deptno NOT IN ( select deptno from emp ) ;
select * from dept where not exists ( select deptno from emp where emp.deptno=dept.deptno) ;

select * from dept where deptno NOT IN ( select deptno from emp where deptno is not null) and deptno is not null;

--结论：10g与空值有关，如果确保非空，可以用到anti的半连接算法
 

 11g：

select * from v$version;

drop table emp purge;
drop table dept purge;
create table emp as select * from scott.emp;
create table dept as select * from scott.dept;
set timing on 
set linesize 1000
set autotrace traceonly explain
select * from dept where deptno NOT IN ( select deptno from emp ) ;
select * from dept where not exists ( select deptno from emp where emp.deptno=dept.deptno) ;

select * from dept where deptno NOT IN ( select deptno from emp where deptno is not null) and deptno is not null;

--结论：11g与空值有关，都可以用到anti的半连接算法，执行计划一样，性能一样

 全局临时表的特性：

--构造基于SESSION的全局临时表（退出session该表记录就会自动清空）

drop table ljb_tmp_session;
create global temporary table ljb_tmp_session on commit preserve rows as select  * from dba_objects where 1=2;
select table_name,temporary,duration from user_tables  where table_name='LJB_TMP_SESSION';

--构造基于事务的全局临时表(commit提交后，不等退出session，在该表记录就会自动清空）

drop table  ljb_tmp_transaction;
create global temporary table ljb_tmp_transaction on commit delete rows as select * from dba_objects where 1=2;
select table_name, temporary, DURATION from user_tables  where table_name='LJB_TMP_TRANSACTION';

　　插入语句：

insert all 
   into  ljb_tmp_transaction
   into  ljb_tmp_session
select * from dba_objects;

　　统计表的记录

select session_cnt,transaction_cnt from (select count(*) session_cnt from ljb_tmp_session),
 (select count(*) transaction_cnt from ljb_tmp_transaction);

　　

 退出来，在进去，就全没有了。